This invention relates generally to optical distance measuring devices and, more particularly, to a method and apparatus for optically measuring the distance to a workpiece by means of sweeping the image space of a lens system by varying the object space of that lens system by utilizing a specular involute surface.
A variety of optical distance measuring devices are known in the prior art. For example, a fiber optic probe having a plurality of transmitting fibers and a plurality of receiving fibers, also known as a bifurcated fiber optic probe, may be used to determine the distance of an object from the end of the probe by measuring the light received by the light receiving fibers. However, the accuracy of this type of probe is dependent upon the reflectivity of the surface being probed and such probes must work very close to the surface being measured.
In accordance with another prior art device, geometric patterns are focused at a pair of discrete axially spaced planes. The geometric shapes form one relatively dark area adjacent to a light or illuminated area. A detector or pair of detectors responsive to the light falling in the dark areas of the patterns generates electrical signals which vary in accordance with the variations in distance of the object to be sensed from each of the planes. In accordance with this prior art device, the signals generated by the detector or detectors corresponding to the two focus planes are compared to provide a signal indicative of the axial spacing of the object from the median distance between the planes of focus.
In all known prior art devices for optical distance measuring, various problems arise since the measurement process is based upon the intensity of the light returned from the target surface and are thus dependent upon the reflectivity of the object to be sensed. While the variations in reflectivity can be compensated, compensation techniques add complexity to the devices and are approximate at best such that high precision cannot be obtained with the systems.
It is, thus, apparent that the need exists for an improved optical distance measuring device which provides an accurate high speed measurement of the positions of a surface within a measurement range regardless of the type of material, surface structure, reflectivity or color of the surface being measured.